Chapter7

　Respiration is an important sign of life and it is the exchange of gas between the body and the environment. It includes three processes: external respiration (pulmonary ventilation and gas exchange in lungs), gas transport in the blood and internal respiration.
　Pulmonary ventilation is the exchange of gas between the alveolus and the environment. The rhythmic contraction and relaxation of the respiratory muscles leads to the expansion or reduction of the chest cavity, which results in the expansion or reduction of the alveoli to change the alveolar pressure. During inspiration, alveolar pressure is lower than the atmospheric pressure so air flows into the lung (inhalation). During expiration, opposite changes occur because the alveolar pressure is greater than the atmospheric pressure and so air flows out of the lung (exhalation). Therefore the original impetus of pulmonary ventilation is respiratory movement and the direct impetus is the pressure difference between alveolar pressure and atmospheric pressure.
　The intrapleural pressure (IP) is very important in the process of pulmonary ventilation. It means the pressure in the pleural space and is made of two factors: alveolar pressure (AP) acting on alveolar cell wall (namely visceral pleura) and recoil force (RF) of lung. They can be expressed as follows: IP=AP-RF. Hence, the intrapleural pressure is a slightly negative pressure. The physiological significance of negative pressure in pleural space is that it is required to hold the lungs in expansion and to enhance venous return and lymphatic return.
　During the process of pulmonary ventilation, only when the driving force overcomes the resistance, can ventilation take place. There are two kinds of resistance to pulmonary ventilation. One is the elastic resistance of pulmonary ventilation and another is the non-elastic resistance of pulmonary ventilation.
　The elastic resistance of pulmonary ventilation can be classified into two categories: alveolar surface tension and elastic recoil of the lung；alveolar surface tension is more important. The common index to reflect the elastic resistance of the lung is the compliance of lung, which is the inverse of elastic resistance. Type II alveolar epithelial cells synthesize and secrete pulmonary surfactant (PS). The physiological functions of PS are to reduce the alveolar surface tension thereby allowing easier lung expansion, stabilizing the different sized alveoli in lungs, preventing alveolar collapse and the infiltration of fluid into the alveoli.
　The non-elastic resistance of pulmonary ventilation includes airway resistance, inertia resistance and tissue viscosity resistance. The airway resistance is a major element of non-elastic resistance. Many factors can lead to contraction of bronchial smooth muscle and the reduction of the bronchi radius. These changes can appear in asthma.
　With regard to how to evaluate the function of pulmonary ventilation, there are many indexes. Timed vital capacity and alveolar minute volume are good indexes for the function of pulmonary ventilation and the efficiency of pulmonary ventilation respectively. Some indexes can be used to differentiate obstructive and restrictive pulmonary ventilation dysfunction.
　The exchange of gases between the alveoli and blood in the capillary vessels is called pulmonary gas exchange. The diffusion constant is proportional to the difference in gas partial pressure, temperature, solubility of the gas and alveoli area；whereas inversely proportional to the respiratory membrane thickness and the square root of the molecular weight. Ventilation-perfusion is another important factor affecting gas exchange.
　In the body, gas exchange with tissues must be carried out through blood transportation. This ,has two transport forms: physically dissolved and chemically combined gases. The physically dissolved form is an obligatory form for oxygen (O2) and carbon dioxide (CO2) into and out of the blood; whereas the chemically combined form is the major form of transportation for oxygen and carbon dioxide. O2 is mainly transported in combination with haemoglobin and CO2 is mainly transported as bicarbonate, both within the erythrocytes.
　Respiratory movements can be voluntary but are mostly generated as automatic rhythmical movements. Cooperation of centres in the medulla and pons provides reflex control of the normal breathing rhythm.
　Respiratory movement is regulated by both mechanical and chemic stimuli. Mechanical stimuli are mainly those evoking the pulmonary stretch reflex (pulmonary stretch reflex includes inflation reflex and deflation reflex and it prevents over expansion of the lungs during strenuous exercise) and the respiratory reflex of respiratory muscle. There are two types of chemoreceptor: peripheral chemoreceptors located in the carotid and aortic bodies and central chemoreceptors located near the ventral surface of the medulla. The peripheral chemoreceptors are sensitive to a decrease in arterial PO2 or pH and an increase in PCO2. When the blood PCO2 rises, CO2 will also rapidly penetrate the blood-brain barrier and enter the cerebrospinal fluid. Subsequently CO2 will promptly be hydrated to produce H2CO3, H+ will dissociate from H2CO3. So the local H+ will stimulate central chemoreceptors in the end. The control centre responds to both sets of chemoreceptors by sending signals to regulate the rate and depth of respiration to maintain the homeostasis of CO2, O2 and H+.